Added GitHub-esque diff syntax highlighting

I thought it might be a good feature add to expand the syntax highlighting to `diff` views. I know that not many people are using Sublime Text for this purpose, but I've found good use for it when I use the [SublimeGit](https://github.com/SublimeGit/SublimeGit) plugin. 

**Example:**

![gst_diffsyn](https://cloud.githubusercontent.com/assets/5373700/2923336/dcffbb60-d713-11e3-9883-fdf2318835e0.png)

Thoughts?